The present invention relates to a speed control apparatus for controlling a driving speed of a hydraulic actuator due to the adjustment of the fluid by a digital valve and, more particularly, to a speed control apparatus of a hydraulic actuator for controlling the driving speed of the actuator so as to become equal to the set speed in a feedback manner.
Hitherto, as a speed control apparatus for controlling the driving speed of the hydraulic actuator due to the adjustment of the flow rate in an analogwise manner, the apparatus as shown in, for example, FIG. 1 is known.
In FIG. 1, a speed setting device 10 consists of a potentiometer and the like and sets the driving speed V.sub.s of an actuator 20 of a hydraulic cylinder and the like to be controlled. Numeral 12 denotes an adding point of the feedback control loop. The adding point 12 outputs the deviation .DELTA.V=(V.sub.s -V) between the set speed signal V.sub.s from the speed setting device 10 and the detection speed V from a speed sensor 22 using a tachogenerator and the like to detect the driving speed of the actuator 20.
A servo amplifier 14 outputs the signal (K.sub.p .multidot..DELTA.V) which is derived by multiplying the deviation signal .DELTA.V from the adding point 12 by K.sub.p by a constant amplification factor, or the signal (K.sub.p .multidot..DELTA.V+K.sub.i .intg..DELTA.Vdt) as the result of the addition of that signal and the signal which is obtained by time-integrating the deviation signal .DELTA.V and further multiplying the result by a predetermined integration constant K.sub.i.
A servo valve 16 converts the electrical signal to a hydraulic pressure and receives the hydraulic pressure from a fluid pressure source 18 such as a hydraulic pump and the like. The servo valve 16 makes the internal torque motor operative in accordance with the driving signal from the servo amplifier 14; converts the electrical signal to the pilot hydraulic pressure by use of the combination mechanism of a nozzle and a flapper; moves a spool by the pilot hydraulic pressure; and adjusts the operating hydraulic pressure to be supplied to the actuator 20 from the fluid pressure source 18.
According to the above feedback control system, the deviation signal .DELTA.V between the set speed signal V.sub.s from the speed setting device 10 and the detection speed signal V to be fed back from the speed sensor 22 is obtained by the adding point 12; the deviation signal .DELTA.V is amplified by the servo amplifier 14 in an analogwise manner; supplies the amplified signal to the torque motor of the servo valve 16; and controls the driving speed of the actuator 20 in a feedback manner such that the deviation .DELTA.V between the set speed V.sub.s and the detection speed V becomes zero by way of the proportional control based on the proportional constant (K.sub.p) set in the servo amplifier 14 or by way of the proportional integration control based on the proportional constant (K.sub.p) and the integration constant (K.sub.i).
FIG. 2 shows a conventional speed control apparatus using a digital control system.
In FIG. 2, a digital speed setting device 24 outputs the set speed V to drive the actuator 20 as digital data of, for instance, eight bits. A rotary encoder 30 outputs a pulse signal of a frequency which is proportional to the driving speed of the actuator 20. The pulse signal from the rotary encoder 30 is input to a speed sensor 32. The speed sensor 32 detects the driving speed V of the actuator 20 from, for example, the reciprocal (1/T) of the period T of the pulse signal and outputs it as the digital speed data V of eight bits.
An operational unit 26 receives the set speed data V.sub.s from the digital speed setting device 24 and the detection speed data V from the speed sensor 32, and obtains the deviation .DELTA.V=V.sub.s -V by a digital arithmetic operation, and further executes the digital arithmetic operation of (K.sub.p .multidot..DELTA.V+K.sub.i .intg..DELTA.Vdt) for, e.g., the proportional integration control. A digital-to-analog (D/A) converter 28 converts the control data which is output from the operational unit 26 to an analog signal, amplifies it by the servo amplifier 14, and outputs the analog control signal to the servo valve 16.
However, in such a speed control apparatus of the hydraulic actuator using the conventional servo valve, the maintenance and management of the operations of the flapper and spool and the like in the servo valve are complicated since sticking and the like are caused by even small particles of dust. Also, the apparatus of the servo valve is large in size and expensive. Furthermore, if it is intended to perform the digital control which can be easily controlled and has a high degree of accuracy, there are the disadvantages such that the D/A converter and the like must be provided and the whole system becomes expensive and is enlarged and the like.